This invention relates generally to vinylidene chloride copolymers containing a processing aid and flexible film produced therefrom. More particularly, vinylidene chloride copolymers of low molecular weight, with a processing aid preferably also of low molecular weight, have been surprisingly found to be of enhanced thermal stability. Thus, they can be extruded into film at a higher temperature, or at conventional temperatures but with less processing aid. The vinylidene chloride copolymer layer in the extruded film can be very thin, as thin as 0.18 mils, yet the film will still possess excellent oxygen barrier characteristics exhibiting an oxygen permeability under about 8 cc/mil/m.sup.2 /day/atmosphere. This is equivalent to an oxygen transmission rate under about 44.44 cc/sq m/day atmosphere for this film having a 0.18 mil gauge vinylidene chloride copolymer layer.
Flexible thermoplastic films made of vinylidene chloride copolymer, hereinafter referred to generally as PVDC (such materials are also commonly known as "saran", which, in the United States has become generic and is not a registered trademark) have been used for a long time to package food products which include cheese, fresh and processed meats, and a wide variety of other food and non-food items. PVDC is a barrier to oxygen and thus such films protect food from spoilage.
Processing aids are standard practice for PVDC to be successfully extruded at commercial rates, i.e. the vinylidene chloride copolymers must be stabilized and plasticized. A successful plasticizer-stabilizer combination is the liquid stabilizer, epichlorohydrin/bisphenol A, an epoxy resin sold as EPON resin 828 by the Shell Chemical Company, and the plasticizer, 2-ethyl hexyl diphenyl phosphate sold as Santicizer-141 by Monsanto Chemical Co. Other known liquid stabilizers include epoxy compounds such as epoxidized linseed oil and epoxidized soybean oil and citrates. A quite successful and satisfactory plasticizer/stabilizer package is made using approximately 4% of Epon 828 and approximately 2% of Santicizer-141 in PVDC. For clarity, it is noted that sometimes the art refers to the epoxy compounds as plasticizers instead of as stabilizers.
A method of producing a multilayer film having a PVDC layer is disclosed in U.S. Pat. No. 4,112,181, issued on Sept. 5, 1978 to Baird, Jr. et al. This patent describes a method of coextruding a tubular film wherein the walls of the tube have at least three layers, a center layer being a PVDC layer. The tubular film is subsequently biaxially oriented by the trapped bubble technique. The 3-layer film may be cross-linked by electron beam irradiation.
Another satisfactory method of producing a multilayer saran film is disclosed in U.S. Pat. No. 3,741,253, issued on June 26, 1973 to Brax et al, which discloses a multilayer, biaxially oriented film having a PVDC barrier layer. This film is made by an extrusion coating process in which a substrate layer or layers of a polymer such as polyethylene or ethylene vinyl acetate copolymer is extruded in the form of a tube, cross-linked by irradiation, and inflated. A layer of PVDC is extrusion coated onto the inflated tubing, and another layer or layers of polymer is simultaneously or sequentially extrusion coated onto the PVDC. After cooling, this multilayer tubular structure is flattened and rolled up. Then, the tube is inflated, and heated to its orientation temperature, thereby biaxially orienting the film. The bubble is rapidly cooled to set the orientation. This process produces a heat shrinkable barrier film with low oxygen permeability. Also, the advantages of a cross-linked film are provided without subjecting the PVDC layer to irradiation which tends to degrade saran. The barrier layer in the examples of the patent to Brax et al is a plasticized copolymer of vinylidene chloride and vinyl chloride.
In Canadian Patent No. 968,689, issued on June 5, 1975 to Gillio-tos et al, the effect of plasticizers such as dibutyl sebacate on the barrier properties of a PVDC barrier layer in a multilayer thermoplastic packaging film is described. First, the Gillio-tos et al patent discloses that homopolymers of vinylidene chloride cannot be converted into film by conventional extrusion techniques because they decompose very rapidly at the temperature of extrusion. Second, by copolymerizing vinylidene chloride with minor amounts of one or more other monomers such as vinyl chloride, methyl acrylate, methyl methacrylate, acryonitile, etc. it is possible to produce copolymers which, when mixed with suitable plasticizers, can be extruded into films which can be oriented by stretching to give heat shrinkable film. The oriented, heat shrinkable, PVDC films are widely used for packaging purposes, particularly for packaging food. As stated in Gillio-tos et al, vinylidene chloride copolymers need to be plasticized so that they can be extruded and stretched into oriented films at commercial rates. The greater the proportion of plasticizer, the lower the viscosity and the easier the polymer is to extrude and orient and the better the abuse resistance of the final product. On the other hand, the oxygen transmission rate of the final product also increases with increasing plasticizer content and for many purposes, especially packaging food, it is vital that the oxygen transmission rate is low. In recent years, the packaging industry has become increasingly demanding and for current commercial practices an oxygen transmission rate below 100 cc/24 hours/m.sup.2 /atmosphere at room temperature is expected and below 50 is highly desirable. The test for oxygen transmission is conducted as per ASTM D3985.
Also, as the thickness of the vinylidene chloride copolymer layer decreases, the oxygen transmission rate will increase. Thus, for thin films, the oxygen transmission rate still should meet the standard of less than 100, preferably less than 50 cc/24 hrs/sq m/atm. As mentioned above, the thin films (0.18 mil vinylidene chloride copolymer layer) of the invention do.
Of interest is U.S. Pat. No. 4,714,638 (Dec. 22, 1987) to Lustig et al, assignors to Viskase. This patent discloses heat-shrinkable, biaxially stretched multi-layer film having a barrier layer of PVDC wherein the comonomer of the PVDC is methyl acrylate in an amount of 5 to 15% by weight. This patent discusses using conventional plasticizers for the PVDC-MA, such as dibutyl sebacate or epoxidized soybean oil. Similar to U.S. Pat. No. 4,714,638 is U.S. Pat. No. 4,798,751 (Jan. 17, 1989), Schuetz, also assigned to Viskase, wherein the PVDC layer of the biaxially stretched, heat-shrinkable multi-layer film is a blend of 2.9-13.5% PVDC-MA and 2.9-11.6% PVDC-VCl. This latter patent also discusses dibutyl sebacate and epoxidized soybean oil.
Also of interest is U.S. Pat. No. 4,320,175 issued March 16, 1982 to Hisazumi et al assignors to Kureha, which shows a PVDC layer composed of PVDC of 0.030 to 0.050 reduced viscosity heat-pressure laminated to a PVDC layer composed of PVDC of 0.050 to 0.075 reduced viscosity. Epoxy compounds are used as stabilizers for the PVDC.
Also of interest are the following patents which show various additives for PVDC. U.S. Pat. No. 4,401,788, issued Aug. 30, 1983 to Hiyoshi et al, assignors to Asahi Dow, shows a PVDC latex with an anionic surfactant, a nonionic surfactant, and a catonic surfactant. U.S. Pat. No. 4,360,612, issued Nov. 23, 1982 to Trumbull et al, assignors to Dow, shows extruded PVDC film containing an alkali metal salt of an alkylaryl sulfonic acid. U.S. Pat. No. 4,418,168, issued Nov. 29, 1983 to Johnson, assignor to Dow, shows stabilizing particulate PVDC by coating thereon a dispersion of tetrasodium pyrophosphate (TSPP) in liquid plasticizer such as epoxidized soybean oil and epoxidized linseed oil. U.S. Pat. Nos. 2,205,449, issued June 25, 1940 and 2,320,112, issued May 25, 1943, both to Wiley, assignor to Dow, show PVDC plasticized with phenoxypropylene oxide (lines 48-49, column 1 of '449) and PVDC plasticized with di-(alpha-phenyl-ethyl) ether (line 16, column 2 of '112). German Patent 3,605,405, priority Feb. 20, 1986, published Dec. 12, 1987, shows 5-layer films with a PVDC core layer, wherein the EVA adhesion layers on each side of the PVDC core layer contain TSPP to heat stabilize the PVDC core layer. U.S. Pat. No. 3,524,795, issued Aug. 18, 1970 to Peterson, assignor to Dow, shows multiple layer PVDC films and mentions at lines 55-59 of column 4 typical plasticizers for PVDC being acetal tributyl citrate, epoxidized soybean oil, and dibutyl sebacate.
Among the plasticizers for thermoplastics which are listed in general articles and in literature references is glycerol or glycerin. Glycerin together with the above-mentioned epoxy resins as a plasticizer combination for PVDC is disclosed in U.S. Pat. No. 4,686,148 to Havens.
An object of the present invention is to improve the thermal stability of vinylidene chloride copolymers and to lessen their tendency to degrade while being extruded, and hence provide improved melt shear stability. An advantage is that PVDC can be extruded at higher temperatures. Alternatively, the PVDC can be extruded at the same conventional temperature, but with less additive to achieve thermal stability. The less the additive is, the lower the oxygen permeability is. Thus another advantage is that films with a thinner PVDC layer can be used and they will have a low oxygen permeability comparable to films with a thicker PVDC layer (and more additive). The after orientation thickness (after biaxial stretching) can be as low as less than about 0.29 mils, preferably less than about 0.26 mils, more preferably less than about 0.23 mils, most preferably less than about 0.20 mils. The limit thickness is about 0.08 mils.